This invention relates to cardiac stimulators which use indicators of patient activity and body position to determine the type and intensity of cardiac stimulation.
The class of medical devices known as cardiac stimulation devices can deliver and/or receive electrical energy from the cardiac tissue in order to prevent or end life debilitating and life threatening cardiac arrhythmias. Pacing delivers relatively low electrical stimulation pulses to cardiac tissue to relieve symptoms associated with a slow heart rate, an inconsistent heart rate, an ineffective heart beat, etc. Defibrillation delivers higher electrical stimulation pulses to cardiac tissue to prevent or end potentially life threatening cardiac arrhythmias such as ventricular fibrillation, ventricular tachycardia, etc.
Early advances in pacing technology have led to a better quality of life and a longer life span. The development of demand pacing, in which the stimulator detects the patient""s natural cardiac rhythm to prevent stimulation during times which the patient""s heart naturally contracts, led to a more natural heart rate as well as a longer battery life.
Another major advance was rate responsive pacing in which the stimulator determines the stimulation rate based upon the patients metabolic demand to mimic a more natural heart rate. The metabolic demand typically is indicated by the patient""s activity level via a dedicated activity sensor, minute ventilation sensor, etc. The stimulator analyzes the sensor output to determine the corresponding stimulation rate.
A variety of signal processing techniques have been used to process the raw activity sensor output. In one approach, the raw signals are rectified and filtered. Also, the frequency of the highest signal peaks can be monitored. Typically, the end result is a digital signal indicative of the level of sensed activity at a given time. The activity level is then applied to a transfer function that defines the pacing rate (also known as the sensor indicated rate) for each possible activity level. Attention is drawn to U.S. Pat. No. 5,074,302 to Poore, et al., entitled xe2x80x9cSelf-Adjusting Rate-Responsive Pacemaker and Method Thereofxe2x80x9d, issued Dec. 24, 1991. This patent has a controller that relates the patient activity level signal to a corresponding stimulation rate. In addition, the controller uses the activity signal over a long time period to determine the adjustment of the corresponding stimulation rates. The activity signal can also indicate when a patient is sleeping to modify the pacing rate as set forth in U.S. Pat. No. 5,476,483 to Bornzin, et al, entitled xe2x80x9cSystem and Method for Modulating the Base Rate during Sleep for a Rate-Responsive Cardiac Pacemakerxe2x80x9d, issued Dec. 19, 1995, which is hereby incorporated by reference in its entirety.
Another method of determining the stimulation rate based upon metabolic need is based upon the body position of a patient. Studies have shown that a patient being upright indicates a higher stimulation rate than for a patient lying down. An example is U.S. Pat. No. 5,354,317 to Alt, entitled xe2x80x9cApparatus and Method of Cardiac Pacing Responsive to Patient Positionxe2x80x9d, issued Oct. 11, 1994. In this patent, the controller monitors a motion sensor to produce a static output which represents the position of the patient, i.e., lying down or upright. This static output is used to determine whether a sleep indicated rate or an awake base rate should be used. However, this system is completely dependent upon the proper orientation of the stimulator housing during implantation for consistent and reliable results.
To further improve the stimulator""s ability to mimic the heart""s natural rhythm, a combination of monitoring both the patient""s activity level and the body position has been envisioned. U.S. Pat. No. 5,593,431 to Sheldon, entitled xe2x80x9cMedical Service Employing Multiple DC Accelerometers for Patient Activity and Posture Sensing and Methodxe2x80x9d, issued Jan. 14, 1997, sets forth a system which monitors both parameters. This patent sets forth a cardiac stimulator which uses a multi-axis DC accelerometer system to monitor both patient position and patient activity. Unfortunately, this accelerometer also depends upon a known orientation during implant and repeated postoperative calibrations for proper operation due to shifting of the stimulator within the implant pocket.
The ability to accurately determine both the patient""s activity level and the patient""s body posture would greatly benefit many patients by providing a more metabolically correct stimulation rate. As well, this combination of sensors could be used to determine the accuracy of other sensors such as PDI, O2 saturation, etc. This enables the controller of the stimulator to blend the outputs of the various sensors to provide the benefit of the each individual sensor. Also, in the case of implantable cardioverter/defibrillators (ICDs), these two outputs would allow modification of the defibrillator thresholds based upon time of day and posture of the patient.
Accordingly, it would be desirable to develop an implantable cardiac stimulator which adjusts the stimulation level based upon the patient activity and the patient body position via sensors that are not device implant orientation sensitive.
The present invention is directed towards an implantable cardiac stimulation device which determines cardiac stimulation levels based upon the patient""s current body position and activity level while eliminating special implantation or calibration procedures. To determine the body position and the activity level, the stimulator monitors the output of a multi-axis DC accelerometer or a combination of sensors to include oxygen saturation, PDI, minute ventilation sensors, etc.
To determine the patient""s current body position, the controller establishes the output of at least two DC accelerometers during times of high activity as the patient""s standing position. Lower activity levels associated with the other body orientations while lying down are also deduced in a similar manner. Then, the stimulator correlates the current outputs of the DC accelerometers with the standing position to determine the current body position and uses the previous and current body positions and the activity level (preferably calculated from the AC acceleration in the anterior-posterior axis, the axis which has the best correlation with the patient activity), to determine the instantaneous stimulation needed.
Because this stimulator depends upon the combination of activity signals and position signals from the multi-axis accelerometer, this device is not dependent upon a predetermined implant orientation or repeated calibration of the accelerometer after implant. If the device should shift within the patient after implant, the controller will accommodate this change during the patient""s next high activity period.
As such, this device monitors the activity and position signals from the multi-axis accelerometer to determine the indicated activity level of the patient and the current body position and then determines the type and intensity of cardiac stimulation the patient needs.
In a further aspect of a preferred embodiment of the present invention, the calculated standing position is monitored to detect changes that may indicate the presence of twiddler""s syndrome.